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If you have the kind that is designed to maintain a desired airflow regardless of duct work, within limits of course, how does it work? I assume there is some kind of pressure sensor, but where is it located? Is it in the motor housing itself? Just curious.

The ECM motors module monitors torque required to turn the motor. Blower curves for specific HP, blower wheel size and static pressure are graphed and configured into the software the ECM motor uses. Each model, size, brand used specific software that is written for the speific unit.

The motor reads the torque and then adjusts accordingly. The actual CFM or static is not read. Some equipment may even offer a readout but it is a conversion from torque.

I should also add that most equipment has a control board with dip switches which configures 24v signals to the module so dehumidification, and other changes can be made without rewriting the software for each application.

Well, I suspect unless the weight of the dirt becomes a factor (enertia), the torque is primarily affected by the air being moved. Variable speed is certainly no cure all for poor ducts or maintenence.